Semiconductors are making rapid progress in capacity, density, integration and surface mounting. Along with this progress, a number of problems are arising in production of semiconductor devices. One of the problems is that the heat or thermal stress applied to a semiconductor chip at the time of surface mounting becomes more severe as the sealing material becomes thinner or depending upon the method of surface mounting. Hence, it is necessary to protect the fine semiconductor circuits from said heat or thermal stress. Meanwhile, in order to form circuits of high density and integration on a semiconductor chip, the technique of multi-layer wiring is indispensable. To achieve it, there is required an interlayer dielectric having high heat resistance, high adhesion and low dielectric constant.
It is known that the above problems can be solved by using a highly heat-resistant polymer such as polyimide or the like, having excellent heat resistance and mechanical properties, as a passivation film, a buffer coat film or an interlayer dielectric. As such a highly heat-resistant polymer for use in semiconductor production, photo-sensitive polyimides have drawn attention in recent years (e.g., Japanese Patent Publication No. 30207/1980 and Japanese Patent Application Laid-Open No. 145794/1979). These polyimide resins have photo-sensitivity by themselves and can give viaholes for wiring, without using any resist. Therefore, in making viaholes, steps of resist coating and its peeling are not required. Further, no harmful chemicals such as hydrazine, halogen-containing solvent and the like are used in its processing, and an ordinary organic solvent is used, which allows safe operation and gives no environmental pollution.
As a noteworthy novel photo-sensitive resin, there can be mentioned positive photo-sensitive polybenzoxazoles constituted by a polybenzoxazole and a diazoquinone compound (e.g., Japanese Patent Publication No. 46862/1989). These resins have high heat resistance, good processability, excellent electrical properties (e.g., low dielectric constant), fine pattern formability and high sensitivity and has a possibility of being used for wafer coating or formation of interlayer dielectric. Further, these positive photo-sensitive resins use an aqueous alkaline solution in forming viaholes, and accordingly require no organic solvent unlike conventional photo-sensitive polyimides, and thus allow operation of higher safety.
Conventional photo-sensitive polybenzoxazoles, however, have a serious drawback in that they have poor adhesion to a substrate, particularly silicon wafer and are peeled from the substrate during development or because of moisture absorption after curing. Further, the film formed after heating and ring closure has slightly insufficient strength and is fragile. As a measure to overcome these drawbacks, it is considered to introduce an adhesion-improving component, a flexibility-improving component, etc. into the main chain of the polymer. However, a polybenzoxazole precursor is actually synthesized by reaction between a dihydroxydiamine and a dicarboxylic acid dichloride; this reaction is effected under narrow reaction conditions; and copolymerization of different components of different reactivities is very difficult to control.
Properties which a photo-sensitive resin composition used for semiconductors must have, include high resolution, high contrast, thick-film formability, etc. Among these properties, high contrast and thick-film formability are greatly dependent upon, in particular, the molecular structure of a photo-sensitive diazoquinone compound. That is, it is required that the diazoquinone compound be soluble in the polymer or its precursor, that the diazoquinone compound can effectively reduce the solubility of the coated film in alkali solution before light exposure, but after light exposure, can heighten the solubility of the polymer or its precursor in alkali solution, and that the photo-sensitive diazoquinone compound after reaction has such a high transparency that the applied light can well reach as far as the bottom of the thick film.
As the photo-sensitive diazoquinone compound, there are generally used esters between 1,2-naphthoquinonediazide-5-sulfonic acid and a phenol, and there are known, for example, polyfunctional photosensitive diazoquinone compounds represented by the following structural formulas [XVI] and [XVII] using a polyfunctional phenol. ##STR1## In these photo-sensitive diazoquinone compounds, however, ultraviolet absorption by the aromatic ring of phenoxy group is not negligibly small, which hindered arrival of light to the bottom of thick film and gave a limit to thick-film formability. Further, when a photo-sensitive resin is used as an interlayer dielectric for multi-layer wiring in semiconductor circuits or as a coating material for semiconductor chips, the photo-sensitive resin is required to have low dielectric constant and low water absorption in view of high-speed operation and reliability. In conventional positive photo-sensitive resins, however, the cured film produced therefrom was insufficient in the above properties because of the polar phenol remaining in the film.
Thus, the conventional positive polybenzoxazoles have excellent properties but simultaneously various serious drawbacks, and as a result find very limited applications.